I looked up Sievierodonetsk on Google Earth to look at roughly how big it is. Without getting super sophisticated, I think we can say with reasonable accuracy that the Russians at Sievierodonetsk have gained an area that is about 10km x 20km (it is actually longer than that, but it is narrower at one side [the south] than the other so calling it 10x20km just makes it easier (I think I'm probably being generous to the Russians with this number).
So, The Battle of Sievierodonetsk: 1 month, 2 weeks, 5 days. 49 days for the Russians to take 200 km^2 of strategically useless territory which is not a strong position for further advance (though the south of Lyschansk is arguably better) and offers little value other than being able to say "100% of Luhansk Oblast is liberated."
Ukraine claims Russian losses just for this battle (Rob recounts UA claims at about
24:00):
4,930 total casualties, 192 tanks, 403 AFVs, 142 artillery tubes, 10 fixed-wing aircraft, 30 helicopters and 102 recon UAVs.
So if that cost-ratio were to hold constant for the rest of Donbas that would be something like (roughly):
0.245 days (per square km)
25 soldiers (per square km)
0.96 tanks
2.01 AFVs
0.71 arty tubes
0.05 aircraft
0.15 helicopters
0.51 UAVs
That is roughly what Russian paid per square kilometer to take control of that ~200 square kms of Sievierodonetsk (assuming we take Ukrainian government at their word . . .).
How much more to go to get the whole Donbas?
Very roughly I think we can say that the remainder of the Donbas region is an approximately equilateral triangle with sides in the 130km ballpark.
The formula for the area of that shape is SQRT(3) / 4 * A^2 (square root of 3 divided by 4 multiplied by A squared [with A being the length of the sides]). (1.73205080757 / 4) = 0.43301270189 130^2 = 16,900 0.43301270189 * 16,900 = 7,317.9 km^2
The Russians have to conquer an additional 7,317 square kilometers to take the rest of Donbas. If we adopt the most simplistic rubric (which is unlikely to be 100% accurate, but about the best rubric we have at hand) of the rate of cost Russia had to pay to conquer Sievierodonetsk, then we can say that taking the rest of Donbas (assuming both Russia and Ukraine's combat power stays where it has been during the battle of Sievierodonetsk) would incur the following costs:
7317*0.245= 1,792.6 days (4.91 years)
7317*25 = 182,925 soldiers
7317*0.96 = 7,024.3 tanks
7317*2.01 = 14,707.2 AFVs
7317*0.71 = 5,195.1 arty
7317*0.05 = 365.9 aircraft
7317*0.15 = 1,097.6 helicopters
7317*0.51 = 3,731.7 UAVs
Of note here: a) TOTAL Russian military was only in the 900,000 ballpark in Jan 2022, with only about 610k ground [CIA World Factbook says "300,000 ground forces"]; b) total tanks was probably only in the ~6k ballpark, though some estimates indicate up to ~10,000 "in storage" . . . 7000 is a lot in any event . . .
There was this famous Greek king named Pyrrhus of Epirus who is famous for having "victorious battles" that were so costly that he wound up losing the war. It would seem that Putin is well on his way to outdoing King Pyrrhus . . . by an order of magnitude or more . . .